Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas.
In the art of photolithography, a photographic material is made imagewise receptive to oily or greasy ink in the photo-exposed (negative working) or in the non-exposed areas (positive working) on a hydrophilic background.
In the production of common lithographic plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used.
Upon imagewise exposure of the light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
On the other hand, EP-A 95202725.8 discloses a negative-working photosensitive imaging element comprising on a hydrophilic surface of a support in the order given (i) a hydrophobic photopolymerizable composition capable of being irradiated with actinic light through the support and/or through the front and containing at least one unsaturated compound with at least one polymerizable ethylenically unsaturated group, at least one hydrophobic thermoplastic polymer and at least one photoinitiator, and (ii) optionally a receptor layer, characterized in that said hydrophobic photopolymerizable composition comprises in the order given (i) a polymerizable layer contiguous to said hydrophilic surface and comprising at least part of said at least one unsaturated compound and (ii) a hydrophobic photosensitive layer contiguous to said polymerizable layer comprising at least part of said at least one hydrophobic thermoplastic polymer and of said at least one photoinitiator and the peeling force of said photopolymerisable composition ranges from 0.1 N/m to 12 N/m.
A particular disadvantage of photosensitive imaging elements such as described above for making a printing plate is that they have to be shielded from the light.
On the other hand, methods are known for making printing plates that are heat sensitive rather than photosensitive. For example, Research Disclosure no 33303 of January 1992 discloses a heat sensitive imaging element comprising on a support a cross-linked hydrophilic layer containing thermoplastic polymer particles and an infrared absorbing pigment such as e.g. carbon black. By imagewise exposure to an infrared laser, the thermoplastic polymer particles are imagewise coagulated thereby rendering the surface of the imaging element and these areas ink acceptant without any further development. A disadvantage of this method is that the printing plate obtained is easily damaged since the non-printing areas may become ink accepting when some pressure is applied thereto. Moreover, under critical conditions, the lithographic performance of such printing plate may be poor and accordingly such printing plate has little lithographic printing latitude.